From a gameplay standpoint, levitation. One single power that upgrades your whole control system in four different ways (better speed, better air control, ability to ride air currents and, later, to damage censors while rolling) is, in my opinion, the best power by default.

Conceptually, though, clairvoyance is the most interesting, and it gives great results even when used on the most unassuming objects (e.g. a keypad).